1. Area of the Invention
The present invention relates to a circuit configuration having a feedback operational amplifier, implemented as fully differential, for amplifying an input signal input differentially into the circuit configuration and outputting the amplified input signal as a differential output signal.
The term “operational amplifier” as defined in the present invention is to be understood very broadly as a configuration capable of amplifying an electrical variable such as a voltage. In particular, it refers to amplifiers, for example, in which a signal applied to the amplifier input is provided having relatively high voltage amplification at the amplifier output. This open amplification (“open loop again”) may be in the magnitude of approximately 104 through 105, for example.
An essential characteristic of the circuit configuration according to the species is that the circuit amplification, i.e., the ratio between output signal and input signal, is practically completely independent of this open amplification (except for transient occurrences) and is solely predefined by an additional (external) configuration or a “feedback network” of the operational amplifier. The feedback network is formed in the circuit configuration according to the species from the totality of coupling, feedback, and decoupling paths.
The feedback network determines the resulting circuit amplification. In the simplest case, the feedback network comprises a configuration of one or more (ohmic) resistors. Alternatively or additionally, other components such as capacitors and/or inductors may be provided to produce the feedback network. Very generally, these components provided to produce the feedback network are thus identified in the following as impedances.
2. Description of the Prior Art
The fully differential implementation of the operational amplifier, which is advantageous for many applications, means that the difference of the potentials applied to the two amplifier inputs (amplifier input voltage) is provided amplified by the open amplification at the two amplifier outputs as the amplifier output voltage, no galvanic coupling between the amplifier inputs and the amplifier outputs being performed by the operational amplifier itself. In order to provide defined potential ratios at the output of such an operational amplifier, in practice, the output common mode voltage is typically set to a predefined value. This output common mode voltage, referred to in short in the following as the “output CM”, is defined as the mean value of the two voltages provided at the amplifier output (each in relation to a reference potential, e.g., a supply potential). Fully-differential operational amplifiers typically have a CM setting input to be impinged by a setting voltage for setting the output CM to a value which precisely corresponds to this setting voltage applied to the setting input.
If the feedback network provides resistive feedback paths from the amplifier outputs to the amplifier inputs, these feedback paths not only influence the resulting circuit amplification, but rather also influence the input-side potential ratios of the operational amplifier through the galvanic coupling which is thus more or less implemented. In many cases, this is a desired effect (or side effect), for example, to set the input common mode voltage, also referred to in the following in short as the “input CM”, to the same value as the output CM. This is true in particular if the input CM was not yet defined using the input-side circuitry of the operational amplifier alone. In this context, it is to be noted that for some operational amplifier circuits, both the input CM and also the output CM are to each be kept reliably in a specific range to ensure perfect function of the operational amplifier. These ranges are at least within the range predefined by two supply potentials of the operational amplifier. For the output CM, it is additionally favorable in regard to a large control range of the operational amplifier if it is approximately in the middle of the range predefined by the two supply potentials. In this case, it is disadvantageous under certain circumstances, however, if the output CM at approximately half of the supply voltage causes an equally large input CM via resistive feedback paths.